Leaderboards for songs, including Weekly Challenge songs, can be accessed anytime.
Search for the song you would like to view the leaderboard for and tap/click to open it.
Click/ tap on the top right hand corner of the song start screen to access the leaderboards.
Your score for the song will be highlighted in the bottom of the screen. You can also toggle between My level, Global and Friends views.
Please note that if you are playing past Weekly Challenge songs, your scores are saved but the Weekly Challenge leaderboard is not updated. You can view your score for the song you played via the Songs tab and the method described above.